The (Na-K)ATPase is the enzyme that is responsible for establishing and maintaining gradients for sodium and potassium across animal cell membranes. These ion gradients are necessary for normal function in nerve cells. The enzyme pumps sodium out of cells and potassium into cells using energy derived from ATP. Very little is known about the structure of this protein and until the structure is known it will not be possible to understand how the protein catalyzes the transport of the ions. The work described in this application will provide structural details of the protein in sufficient detail to trace the pathway of each polypeptide through the membrane in which it is found. This will be done by site-specific cleavage of the protein, alignment of the resulting fragments into a fragmentation map of the polypeptide, and identification of those regions of each polypeptide that are either on one side of the membrane or the other, or are embedded within the interior of the membrane. A new reagent will be synthesized to facilitate this identification. Upon completion, this work, together with kinetic data on transport, will provide a good description of these transport systems.